1. Field of the Invention
The invention relates to heat engineering and, more particularly to tubular air heaters for steam boilers, furnaces and similar apparatus, employed mainly for large plants.
2. Description of the Prior Art
There are known tubular air heaters, wherein gases and air flow in a criss-cross pattern with respect to one another. The tubular air heaters for large plants are generally built, to suit the available space, in two-pass arrangements with respect to air flow. The criss-cross flow, which ordinarily presents a poor utilization of the available temperature head, gives rise in the two-pass air heaters to particularly high temperature head losses, attaining 30% and over.
There is known a method for enhancing the temperature head in the air heaters with criss-cross flow, consisting in using a Z-criss-cross flow, wherein air is conveyed to each pass from one and the same side, i.e. of one and the same direction. If there is no leakage of air between the passes, the Z-criss-cross flow performance is substantially improved. The Z-criss-cross flow air heaters are, as a rule, multiple flow ones, i.e. located in parallel gas ducts.
Disadvantages of the Z-criss-cross multiflow air heaters are that they are complicated and the bypass air conduits are bulky, where the provision of internal partitions adequately tight to prevent air leakage between the passes is a problem.